The mixture identification system is used, for example, to detect foreign matter admixed in a material, and such detection apparatus is disclosed in Patent Document 1 identified below. In the apparatus disclosed in Patent Document 1, near-infrared light is irradiated onto tobacco leaves as a material, namely, tobacco material, and the reflected light from the tobacco material is captured using a near-infrared CCD camera to obtain image data. The image data is processed, and based on the processing results, foreign matter admixed in and having nearly the same color as the tobacco material is detected.
[Patent Document 1] Unexamined Japanese Patent Publication No. 2002-28544 (see [0016] to [0019] and FIG. 1)
More specifically, the detection apparatus of Patent Document 1 extracts near-infrared light with specific wavelengths (1.58 μm, 1.73 μm) from the infrared light reflected from the tobacco material and, based on the reflectance derived from the extracted near-infrared light, determines whether the object that reflected the extracted near-infrared light is the tobacco material or foreign matter. The extracted near-infrared light exhibits distinctive reflectance with respect to the tobacco material, and therefore, this technique cannot be applied to detection of foreign matter admixed in materials other than the tobacco material. Accordingly, the detection apparatus of Patent Document 1 lacks versatility.
Also, the detection apparatus of Patent Document 1 includes a spectroscope for extracting, that is, separating near-infrared light with the specific wavelengths from the light reflected from the tobacco material. The spectroscope includes a prism and a plurality of optical filters. With this type of spectroscope, it is not easy to modify the specification of the spectroscope when near-infrared light with wavelengths different from the specific wavelengths is to be extracted.
Further, the near-infrared CCD camera is adapted to intermittently acquire an image of the tobacco material while the tobacco material is being conveyed, and the image data thus obtained is processed separately on a frame-by-frame basis. Accordingly, in order to detect foreign matter admixed in the tobacco material without fail, the speed of conveying the tobacco material must be slowed, requiring much time for the detection of foreign matter. The detection apparatus of Patent Document 1 is therefore not suited for materials whose foreign matter needs to be detected at higher speeds.
When the tobacco material is imaged frame by frame, moreover, near-infrared light is irradiated unevenly onto the tobacco material within a frame, and if part of the tobacco material is shaded, then foreign matter cannot be detected with accuracy. It is, however, not easy to irradiate near-infrared light uniformly onto the tobacco material within a frame.